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Can anyone explain how the variable valve timing works? I have read statements like, "the other cam comes online at 6000 rpm," but I don't understand how the cam changes. Any insight would be greatly appreciated.
 

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From Toyota:

Based on the VVT-i system, the VVTL-i system has adopted a cam changeover mechanism that varies the amount of lift of the intake and exhaust valves while the engine is operating at high speeds. In addition to achieving higher engine speeds and higher outputs, this system enables the valve timing to be optimally set, resulting in improved fuel economy.


When the engine is operating in the low- to mid-speed range, the low- and medium-speed cams of the camshafts operate to move the two valves via the rocker arms. Then, when the engine is operating in the high-speed range, the signals from the sensors cause the engine's control module (ECM) to change the hydraulic passage of the oil control valve (for the variable valve lift), thus changing to the operation of the high-speed cams. Now the lift of the intake and exhaust valves increases, allowing the introduction of a greater volume of air-fuel mixture, as well as the discharge of a greater volume of exhaust gases. As a result, the engine operates at higher speeds and higher outputs when the engine is under more strain.
For pictures and a more detailed explanation, check out this site.
 

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VVTL-i is composed of two systems:

VVT-i, which acts as a continuously variable cam gear on the intake cam. So, intake cam timing is constantly being adjusted as the engine runs. Many/most new cars have similar systems, some acting on both cams.

... and VVL, which for almost all intents and purposes is identical to good old DOHC VTEC. There are extra lobes on both the intake and exhaust cams, which sophisticated rocker arms switch to high-rpm.

Just to be clear, there is no actual extra camshaft, just extra lobes. But slang commonly refers to them the big and little cams. :)
 

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performance and fuel mileage

That's a good explanation of the mechanism. The consequences are an engine that is great to live with for "normal" driving, but has the guts you need when you want to move.

My 2003 Pontiac VibeGT shares this engine -- and the 6-speed -- with its Matrix cousin and the Celica GT model. It is great fun to drive around, but even more fun on a long trip. Last summer I took a trip from Pittsburgh to British Columbia and back, about 5500 miles over all kinds of terrain.

It was my best western trip ever, of many. This engine, even in the probably 3000 lb. VibeGT, will move if you need to. But just cruising along at 70 to 80 mph, I was getting between 33 and 38.5 mpg on every tank.

When I saw that Lotus has opted for this engine, I jumped on the list at Ascot Motors near Pittsburgh. This will be best of both worlds -- a British sports car with a Toyota engine! I've seen listings of mileage for the Elise at 30. Of course, it depends a lot on how you drive, but I get 30 regularly around town in my Vibe. Pulling a half ton less weight around, I expect somewhat more in the Lotus. Of course, I may be tempted to drive a bit faster.
 
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